Abstract Estrogen receptor alpha (ER?) acts to integrate systemic signals from estrogens and growth factors to control numerous aspects of development and homeostasis within the organism. In response to acute exposure to ligand binding ER? is degraded via the 26S proteasome pathway. However, in order for continued estrogen responsiveness to be maintained during the reproductive, premenopausal stage mechanisms must exist to regulate steady state protein levels of ER?. Little is known about the regulation of ER? steady state levels. Unexpectedly, using multiple approaches including various mouse models and mammary gland transplants we have identified a novel mechanism in which physical association of the Ser/Thr protein kinase, RSK2, with ER? regulates the steady state protein levels of ER? in the adult mammary gland. Furthermore, the ability of RSK2 to regulate ER? levels was not confined to the mammary gland as we observed decreased ER? levels in the uteri and liver of adult female mice in which RSK2 was knocked out. We hypothesize that the RSK2/ ER? complex regulates the steady state protein levels of ER? in vivo to maintain estrogen-responsiveness in the adult. To further investigate our hypothesis we will focus on the mammary gland because mammary gland regeneration experiments in combination with altering gene expression greatly facilitates mechanistic studies. We propose to test our overall hypothesis through these Aims. In Aim 1 we will identify the mechanism by which RSK2 stabilizes ER? protein levels in vivo. In Aim 2 we will identify how the RSK2 contributes to estrogen responsiveness in vivo. Data generated in this proposal will be analyzed using the appropriate statistics for end point and longitudinal analysis. The outcome of these experiments will increase our understanding of the mechanisms that regulate the functions of ER? in normal homeostasis and will provide insight into disruption of this process that can result in estrogen-dependent cancers and metabolic dysfunction.